How Comets Cause Meteor Showers

A 2010 Orionid meteor, seen over Western Ontario, Canada. A waxing gibbous moon shines brightly at the left side of the image.Credit: NASA courtesy of Meteor Physics Group, University of Western Ontario

Meteors ? popularly referred to as
"shooting stars" ? are generated when debris enters and burns up in
Earth?s atmosphere. But astronomers have found that some meteor swarms
correspond closely to the orbits of known comets.?

In the wake of the recent close pass of Comet
Hartley 2,
it is interesting to note that our Earth is currently interacting with the
cosmic leftovers from a far more famous icy wanderer: Halley's Comet.

And more meteors spawned by comets are expected to give
skywatchers a cosmic show in upcoming weeks.

Halley?s legacy

Right now, in fact, we are sweeping through some of the
dust that was shaken
loose from Halley?s Comet as it runs along its gigantic orbital
loop from the vicinity of the sun all the way out to Neptune.

These tiny comet crumbs ram into our atmosphere at high
speeds to create shooting stars, which we call the Orionid
meteor shower.

There are actually two points along Halley?s path, where
it comes relatively near to our orbit. One of these points corresponds to early
May and causes a meteor display that emanates from the constellation Aquarius,
the Water Carrier. The other point lies near the late October part of our orbit
to produce the Orionids.

In May we meet the "river of rubble" shed by
the comet on its way outward from its nearest approach to the sun, while in
October we encounter the part of the meteor stream moving inward toward the
sun.?

The meteors are moving through space opposite or contrary
to our orbital direction of motion. That explains why both the Aquarids and the
Orionids hit our atmosphere very swiftly at 41 miles (66 km) per second ? only
the November
Leonids move faster.??????

Another distinguishing characteristic that October's
Orionids share with the May Aquarids is that they start burning up very high in
our atmosphere ? possibly because they are composed of lightweight material.

This means they likely come from Halley's diffuse surface
and not its core.

Moon muscles in on meteor show

Unfortunately, 2010 is a poor year for the Orionids
thanks to the presence
of the moon, which turns full today (Oct. 22).

It will flood the sky all night long with its brilliant
light right on through the weekend. So although the Orionids are currently near
their peak, producing hourly rates of around 20 per hour, most of these streaks
of light will likely be obliterated by the bright moonlight.?

Still, an exceptionally bright Orionid, darting from out
of the region of the constellation Orion (from where we get the name
"Orionid"), might still be glimpsed.?

The best time to look would be in the early hours of the
morning; the stars of Orion dominate the southern sky around 4 a.m. local time.
After this weekend, Orionid activity will begin to slowly descend. The last
stragglers usually appear sometime in early to mid-November.?

But by then, the moon will no longer be a factor and
another meteor shower will take center stage.

The Taurids arrive

The Taurid meteors, sometimes called the ?Halloween
fireballs,? show up between mid-October and mid-November. But between Nov. 5-12
will likely be the best time to look for them this year, taking into account
both their peak of activity and the fact that the moon will be new on Nov. 5,
offering no interference whatsoever.

Even when the moon widens to its first quarter phase on
Nov. 13, it will still set before midnight, leaving the rest of the night dark
for meteor watchers.

After the moon sets, some 10 to 15 meteors may appear per
hour. They are often yellowish-orange and, as meteors go, appear to move rather
slowly.

The Taurid meteor shower's name comes from the way these
meteors seem to radiate from the constellation Taurus, the Bull, which sits low
in the east a couple of hours after sundown and is almost directly overhead by
around 1:30 a.m.

In the case of the Taurids, they are attributed to debris
left
by Comet Encke, or perhaps by a much larger comet that upon
disintegrating, left Encke and a lot of other rubble in its wake.

The Taurids are actually divided into the Northern
Taurids and the Southern Taurids. This is an example of what happens to a
meteor stream when it grows old.?

So what was originally one stream now diffuses into a
cloud of minor streams and isolated particles in individual orbits. These cross
Earth's orbit at yet more widely scattered times of the year and coming from
more scattered directions until they are entirely stirred into the general haze
of dust in the solar system.?

The two radiants lie just south of the Pleiades. So
during the first couple of weeks of November, if you see a bright, slightly
tinted orange meteor sliding rather lazily away from that famous little smudge
of stars, you can feel sure it is a Taurid.?

Mark your calendars

Because its orbit passes rather close to Earth's you
might be wondering if Comet Hartley 2 can produce any kind of meteor display.
The answer is yes, although at the present time, we could hardly call it a
meteor shower.?

We currently pass closest to the orbit of Hartley 2
around Nov. 2, but at the moment our respective orbits are too widely separated
to produce much more than a few meteors during the course of an entire night?s
watch.?

However, if the calculations of Russian astronomer
Mikhail Maslov are correct, Hartley 2 is destined to provide a spectacular
meteor display for us in the distant future.

Using a method which he calls "vertical
trails," Maslov forecasts that in the year 2055, we will encounter
considerable amounts of comet debris shed by Hartley 2 from 1985 right on
through the first half of the 21st century.

They will appear to emanate from the constellation of
Cygnus, the Swan, not too far away from the beautiful double-star Albireo. This
could turn out to be a true "storm" of meteors, producing anywhere
from 1,000 to 1,300 per hour.?

Especially to all of our younger readers, I can only say,
mark your calendars!

Joe Rao serves as an instructor and guest
lecturer at New York's Hayden Planetarium. He writes about astronomy for The
New York Times and other publications, and he is also an on-camera
meteorologist for News 12 Westchester, New York.